1. Field of the Invention
This invention relates to protection circuits and particularly to protection circuits for electronic devices capable of using a plurality of different power sources.
2. Description of the Related Art
Electronic devices (including, for example, desktop computers, servers, portable computers, palmtop computers, personal digital assistants (PDAs), and celluar telephones) need to be protected from component-damaging high currents that can develop from short circuits ("absolute shorts" or "shorts") or soft-short circuits ("soft-shorts") that develop in the device. Soft-short circuits occur when there is some impedance in the conductive path of the device, but the impedance is lower than the device is designed for, thereby leading to excessive current flow, or an over-current condition. A variety of devices can be used to protect against the over-current conditions resulting from shorts or soft-shorts, including files and polymeric positive temperature coefficient switches ("polymeric PTC" or "poly" switches). Fuses are the lowest cost (and lowest power consuming) solution, but they suffer from slow response time, frequent spurious operation, one-time use, and they often subject sensitive electronic components to high instantaneous power absorption. Poly switches protect devices from over-current conditions because the switch increases resistance ("trips") as it is heated by the current. The device remains tripped as long as the over-current is supplied, but will reset when the over-current is removed. Although, poly switches can be reset, poly switches have response times and holding current requirements that are still too large for many applications, particularly use with power metal-oxide-semiconductor field effect transistors (MOSFETs) for power switching in electronic devices.
Instrumentation amplifiers and high side discrete differential current sensing devices can be used in place of fuses and poly switches. High side current sensing is faster than using a fuse or poly switch, and is particularly desirable in electronic devices that do not have a unified ground return e.g., a laptop computer system with several batteries and a DC power source utilizing a moted or pocketed grounding configuration. High side current sensing is also advantageous in situations where a sensitive ground reference cannot be disturbed with a sense resistor that could create voltage offsets, and because ground referenced schemes cannot protect against bypass capacitor failure.
However, instrumentation amplifiers and high side discrete differential current sensing devices suffer from their own set of drawbacks. Such components are generally high cost, they have high quiescent current drain (a particular problem in battery powered electronic devices), they lack the capability to protect during absolute short circuit conditions, and they lack the capability to ignore transient events, (such as power source transitions in devices which can utilize more than one power source, and in-rush current events). For example, in a portable computer system having both a rechargeable battery and a connection to an external DC source (e.g., an AC-DC adapter), large in-rush currents can occur when a power source selector circuit switches the power source from the low voltage, discharged battery to the relatively high voltage DC source. Similar in-rush currents can be generated when switching from a low voltage, discharged battery to a higher voltage, fully charged battery.
Accordingly, it is desirable to have a protection circuit that functions during both absolute and soft-short conditions, has low quiescent power drain, and is capable of handling transient events such as in-rush currents. Additionally, it is desirable to have a protection circuit that can reduce in-rush current, and pretest an electronic device for shorts before allowing power to be applied to the device.